Hydrogen ion measurement

During arterial blood gas analysis, representation of quantity of hydrogen ions

present in the sample may be displayed as pH, hydrogen ion concentration or both.

The following statements are correct equivalences EXCEPT which one?

a) pH 7.6 = 25 nanomol/L

b) pH 7.4 = 40 nanomol/L

c) pH 7.3 = 50 nanomol/L

d) pH 7.2 = 63 nanomol/L

e) pH 7.0 = 114 nanomol/L

Answer: e

Explanation

This question might seem unreasonable but in fact tests recollection of the fact that

pH is the negative logarithm to the base 10 of the hydrogen ion concentration. Where

the pH is an integer it follows that the hydrogen ion concentration must be a multiple

of ten (it is simply ten raised to the power of that integer (as a negative) i.e. pH 7

is 10−7 mol/L = 100 x 10−9 mol/L = 100 nanomol/L) so Option (e) is quite clearly

incorrect. In fact a hydrogen ion concentration of 114 nanomol/L has a pH of 6.94.

Just as a clinician might be challenged by working in a clinical area where mmHg

are used instead of kPa for expressing gas tensions, an anaesthetist must be familiar

with working in pH or nanomol/L as an expression of hydrogen ion concentration.